WCDMA Transmitter

1 WCDMA Transmitter

1 WCDMA Transmitter

1/ 038 02 - EN/LZU 108 6245 PA3 - 1 -

WCDMA Radio Workshop

Intentionally Blank

- 2 - 01/ 038 02-EN/LZU 108 6245 PA3

1 WCDMA Transmitter

Objectives

Upon completion of this chapter the student should be able to:

Explain the basic structure of a WCDMA transmitter

List the different error protection schemes used in WCDMA

Explain how spreading is performed

Explain how scrambling is performed

Explain how channels are added in the downlink

Explain the principle of Quadrature Phase Shift Keying (QPSK)

01/ 038 02-EN/LZU 108 6245 PA3 - 3 -


Intentionally Blank

- 4 - 01/ 038 02-EN/LZU 108 6245 PA3

1 WCDMA Transmitter

CONTENTS

INTRODUCTION..............................................................................6

ERROR PROTECTION....................................................................7

CYCLIC REDUNDANCY CHECK (CRC)...................................................7

FORWARD ERROR CORRECTION (FEC)................................................8

INTERLEAVING..........................................................................................9

SERIAL TO PARALLEL CONVERSION.......................................10

SPREADING..................................................................................11

ORTHOGONAL CODES (OC)..................................................................12

SCRAMBLING...............................................................................15

SCRAMBLING CODE GENERATION......................................................15

SPREADING AND SCRAMBLING EXAMPLE..............................18

CHANNEL ADDITION....................................................................19

Modulation......................................................................................20

01/ 038 02-EN/LZU 108 6245 PA3 - 5 -


INTRODUCTION

A simplified diagram of the WCDMA downlink transmitter from an RBS is shown in Figure 1-1 below.

1/ 038 13 - EN/LZU 108 5418 PA1 WCDMA Radio Wor kshop5

RBS WCDMA Transmitter

Pulse shaping

Pulse shaping

+

Cos (ωt)

- Sin (ωt)

Real

&

Imag

split

Σ

Cch,SF,m

Serial

to

Parallel

Sdl,nI

Q

Error Protection

I + jQ

G1

CH1 +

Gn

CHn

Error Protection

Spreading ScramblingChannel addition

Modulation

bit rate (kbps) symbol rate (ksps) chip rate (3.84 Mcps) RF (2 GHz)

I

Q

Figure 1-1 RBS WCDMA Transmitter

A larger picture of this can be found at the back of this book.

The data arriving from the RNC goes through the five processes listed below:

Error Protection

Spreading

Scrambling

Channel Addition

Modulation

These processes are explained in this chapter.

- 6 - 01/ 038 02-EN/LZU 108 6245 PA3

1 WCDMA Transmitter

ERROR PROTECTION

To cope with bit errors introduced typically during the transmission across the air interface, the data is subjected to the following error protection processes:

Cyclic Redundancy Check (CRC)

Forward Error Correction (FEC)

Interleaving

CYCLIC REDUNDANCY CHECK (CRC)

Cyclic Redundancy Check (CRC) is used to detect if there are any uncorrected errors left after error correction.

Blocks of data are passed through a CRC generator, which will perform a mathematical division on the data producing a remainder or checksum. This is added to the block of data and transmitted.

The same division is performed on the data block in the receiver. If a different checksum is produced the receiver will know that there is an error in the block of data (alternatively there is an error in the received checksum). Based on this knowledge the receiver has two choices:

1) Discard the block if it is a voice transmission or

2) Ask for a retransmission in the case of packet data.

The longer the checksum, the greater is the accuracy of the process. It could be imagined that various combinations of errors on the data and the checksum would produce the same checksum. The longer the checksum the less likely it is for this to happen.

WCDMA specifications specify a range of checksum lengths ranging from 0 to 24 bits. PKzip, used to compress files in the computer industry uses a 32-bit checksum for greater accuracy.

01/ 038 02-EN/LZU 108 6245 PA3 - 7 -


FORWARD ERROR CORRECTION (FEC)

The next block in the transmitter is Forward Error Correction. This is sometimes called ‘channel coding’. The function of this block is to help the receiver correct bit errors caused by the air interface.

One method for correcting these errors would be to send the information a number of times. Provided this is more than twice, the receiver could select which message is most correct by a “best out of three” decision. The more times the data is transmitted the better is the error protection. However the bandwidth is also increased proportionally

What is required is a system that provides forward error correction with minimal increase in bandwidth.

There are two basic types of FEC available, Block or Continuous codes.

Block Codes work by processing the data into unique code words. This would be similar to transmitting “New York City” to represent ‘NYC’. These redundant bits provide the error correction. As this type of system works on blocks of data it is not suitable for conversational transmissions.

Continuous codes, such as Convolutional Codes and Turbo Codes, on the other hand, are continuously produced as the data is fed to the FEC device. The result will contain redundant bits that help to correct errors.

IS-95, CDMA2000 and WCDMA utilize Convolutional Coding, for low data rates where a low latency and real time processing are required. For high data rate services where latency and processing power is not a problem Turbo Coding is used. This type of coding gives a much better error correction performance than traditional methods.

When Convolutional Coding is applied at the transmitter, a Viterbi decoder is used to recover the original data at the receiver.

The data rate is increased by a factor of two in the case of Convolutional Coding rate ½ and by a factor of 3 when Turbo or Convolutional Coding rate 1/3 is employed.

There is also a small additional overhead added by CRC and tail or trellis termination bits required before FEC.

- 8 - 01/ 038 02-EN/LZU 108 6245 PA3

1 WCDMA Transmitter

INTERLEAVING

A radio channel produces bursty errors. Because Convolutional and Turbo coding are most effective against random errors, interleaving is used to randomize these bursty errors. The interleaving length is determined by the delay requirements of the service. Speech service, for example, requires a shorter delay than data services.

The interleaving lengths supported in the WCDMA RAN are 10, 20, 40 and 80msec.

Figure 1-2 below, shows some examples of error protection used in the WCDMA RAN.


• CRC 12

• Convolutional Coding (Rate1/2 and 1/3)

• 20 msec InterleavingSpeech

• CRC 16

• Turbo Coding (Rate 1/3)

• 20 msec Interleaving

CS 64

• CRC 16

• Turbo Coding (Rate 1/3)

• 10 msec Interleaving

Interactive

Error Protection

CHn

Data rate is increased by a factor of 2 (Rate

1/2) or 3 (Rate (1/3)

plus

overhead added by CRC and Convloutional

or Turbo coding

Figure 1-2 Error Protection Examples

01/ 038 02-EN/LZU 108 6245 PA3 - 9 -


SERIAL TO PARALLEL CONVERSION

Since I/Q modulation is used, the data is split into an ‘I’ and ‘Q’ stream. The mapping is such that even and odd numbered bits are mapped to the I and Q branch respectively. This process has the effect of halving the rate of the data on each branch. The data ‘bits’ on the I and Q branches are referred to as ‘symbols’ and measured in ksymbols per second (ksps). This reduction in data rate is illustrated in Figure 1-3 below.


Serial to Parallel Conversion

Serial

to

Parallel

I

Q

kbits per second (kbps)

Eg. 60 kbps

ksymbols per second (ksps)

Eg. 30 ksps

Figure 1-3 Serial to Parallel Conversion

In the example in Figure 1-3 above the result of the process is that two 30 ksps streams are produced from the input 60 kbps.

- 10 - 01/ 038 02-EN/LZU 108 6245 PA3

1 WCDMA Transmitter

SPREADING

The symbols from the Serial to Parallel Convertor are spread to the chip rate (3.84 Mcps) by a Channelization Code (Cch,SF,m). This is illustrated in Figure 1-4 below.


Spreading

Cch,SF,m

I + jQ

+Symbol rate (xx ksps)

Chip rate (3.84 Mcps)

Figure 1-4 Spreading

The chip rate of the channelization is fixed at 3.84 Mcps but the symbol rate will vary depending in what channel is being spread. An example of a 960 ksps channel being spread is illustrated in Figure 1-5 below.


Spreading Example

Symbols are spread to the chip rate by Channelization Code

Spreading Factor (SF) = Chip Rate/Symbol rate

1 -1 1 1 -1Symbols

@960 ksps

1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1chips

SF=4

1 -1 1 -1 1 -1 1 -1 -1 1 -1 1-1

1x-1=-1

1

1x 1= 1

-1

1x-1=-1

-1 1 -1 1

-1x 1= -1

-1x-1= 1

-1x 1= -1

-1x-1= 1

Result 1

1x 1= 1

X

Figure 1-5 Spreading Example

In this example there are four chips representing one bit of data. In other words the Spreading Factor (SF) is 4.

01/ 038 02-EN/LZU 108 6245 PA3 - 11 -


The formula below can be used to calculate the SF when the symbol rate is known:


Spreading Example

Symbols are spread to the chip rate by Channelization Code

Spreading Factor (SF) = Chip Rate/Symbol rate

1 -1 1 1 -1Symbols

@960 ksps

1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1chips

SF=4

1 -1 1 -1 1 -1 1 -1 -1 1 -1 1-1

1x-1=-1

1

1x 1= 1

-1

1x-1=-1

-1 1 -1 1

-1x 1= -1

-1x-1= 1

-1x 1= -1

-1x-1= 1

Result 1

1x 1= 1

X

ORTHOGONAL CODES (OC)

Channelization codes are said to be orthogonal. This means that when represented as vectors the will always be at right angles to each other. These are ideally suited for providing channel separation of spread channels, as it is not possible for any channel to corrupt all the chips of another when added together, regardless of power imbalance. This is illustrated in the example of addition of a SF 128 channel (Ch1) and SF 8 channel (Ch2), illustrated in Figure 1-6 below.


Channelization codes are ‘Orthogonal’

CH1

(SF 128)

(1,1)

CH2

(SF 8)

(1,-1)

(1,-1)

Ch1 + CH 2(1,-1)

CH1

(SF 128)

CH2

(SF 8)

Lost one chip= 1/128 symbols

Since codes are Orthogonal one channel cannot currupt both chips, regardless of

the power imbalance

Figure 1-6 Channelization codes are ‘Orthogonal’

- 12 - 01/ 038 02-EN/LZU 108 6245 PA3

1 WCDMA Transmitter

Figure 1-7, below shows the orthogonal code tree. Two codes are said to be orthogonal when their inner product is zero. The inner product is the sum of all the terms we get by multiplying two codes element by element.


Orthogonal Code Tree

Chip Rate = 3.840 Mcps

480 ksp/s 480 ksp/s 480 ksp/s 480 ksp/s 480 ksp/s 480 ksp/s 480 ksp/s 480 ksp/s

1

1-1 11

1-11-1 1-1-11 11-1-1 1111

1-11-11-11-11-11-1-11-11 1-1-111-1-111-1-11-111-1 11-1-111-1-111-1-1-1-111 1111-1-1-1-1 11111111

Figure 1-7 Orthogonal Code Tree

For example, (1, 1, 1, 1) and (1, 1, -1, -1) are orthogonal since

(1 * 1) + (1 * 1) + (1 * -1) + (1 * -1) = 0

The code tree corresponds to different discrete Spreading Factor (SF) levels, SF=1, 2, 4, 8…(2n). Different spreading factor levels mean different code lengths, and they are therefore normally referred to as Orthogonal Variable Spreading Factors (OSVF). The idea is to be able to combine different messages with different spreading factors and keep the orthogonality between them. We therefore need codes of different lengths that are still orthogonal. Of course, the chip rate remains the same for all codes, so short ones will be transmitted at a higher information rate than longer ones. The longer the code is the lower will the data rate. The spreading factor corresponds to the length of the code and the number of channels sending at a certain bit rate.

SF: 4-512 is allowed in the WCDMA Downlink.

SF: 4-256 is allowed in the WCDMA Uplink.

01/ 038 02-EN/LZU 108 6245 PA3 - 13 -


The Channelization Code naming scheme from the 3GPP Technical Specification no. 25.201 is illustrated in Figure 1-8 below.


Channelization Code naming scheme

Designation: cch, SF , code number

1

1 -1

1 1

1 1 1 1

1 1 -1 -1

1 -1 1 -1

1 -1 -1 1

C1,0

C2,0

C2,1

C4,0

C4,1

C4,2

C4,3

SF = 1 SF = 2 SF = 4

3GPP TS 25.2013GPP TS 25.201

Downlink = SF 4 ----------------> SF 512

Uplink= SF 4 -----------> 256

Figure 1-8 Channelization Code naming scheme

- 14 - 01/ 038 02-EN/LZU 108 6245 PA3

1 WCDMA Transmitter

SCRAMBLING

The spread data is scrambled with a cell specific Scrambling Code (Sdl,n) to separate it from other cells in the area. This is illustrated in Figure 1-9 below.


Scrambling

Sdl,n

I + jQSpread symbols Scrambled symbols

Cell specific Scrambling code

Figure 1-9 Scrambling

SCRAMBLING CODE GENERATION

Scrambling Codes are created from Pseudo Noise (PN) sequences generated using a Shift Register as illustrated in Figure1-10 below.


Scrambling Code Generation

Scrambling Codes are created from Pseudo Noise (PN) sequences which are generated using a Shift Register

D D

clock

D D

1 2 3 N

1010010010001110101...

Figure 1-10 Scrambling Code Generation

01/ 038 02-EN/LZU 108 6245 PA3 - 15 -


Shift register sequences are not orthogonal, but they do have a narrow autocorrelation peak. By using a single shift register, maximum length (M) sequences can be obtained. Such sequences can be created by applying a single shift register with a number of specially selected feedback taps. If the shift register size is (n) then the length of the code is equal to 2n-1. The number of possible codes is dependent on the number of possible sets of feedback taps that produce an M sequence.

The mathematics of these generators is equivalent to the operation of ordinary algebra applied to abstract polynomials over an indeterminate X, with binary valued coefficients. Each sequence is based on a generator polynomial

G(X)= bnXn + bn-1Xn-1 + bn-2Xn-2 +……+ b1X1 + 1

Figure 1-11 below, illustrates auto- and cross correlation plots for a 2000 chip length PN code.


PN Code Correlation Plots

Autocorrelation of 2000-bit PN sequenceTime offset = 0

-800 -600 -400 -200 0 200 400 600 800 1000-100

0

100

200

300

400

500

600

Single, centered correlation peak indicates that two signalsare identical, with zero time offset

Cross-correlation of two different PN sequences

-800 -600 -400 -200 0 200 400 600 800 1000-100

0

100

200

300

400

500

600

Figure 1-11 PN Code Correlation Plots

As can bee seen from the auto-correlation plot, a peak is only produced when both codes are perfectly aligned, any slippage produces noise. When two different PN codes are correlated as is the case in the cross-correlation plot, it can bee seen that only noise is produced regardless of the time alignment of the codes.

Because of this property, it is possible to distinguish transmitters in a WCDMA network regardless of their phases.

- 16 - 01/ 038 02-EN/LZU 108 6245 PA3

1 WCDMA Transmitter

For this reason SCs are used to distinguish transmitters, that is cells in the downlink and UEs in the uplink, as illustrated in Figure 1-12 below.


Use of Scrambling codes in the network

SCa SCb

SCc SCd

SC1 SC1

SC2 SC2

Uplink: 16,777,216 Scrambling codes used to distinguish each UE Downlink: 512 Scrambling codes used to distinguish each cell

Figure 1-12 Use of Scrambling codes in the network

In the WCDMA RAN there are 16,777,216 SCs available for distinguishing UEs in the uplink and 512 SCs used to distinguish cells in the downlink.

An example of the error protection scheme, scrambling and channelization codes taken from the TIW L3 Presentation window is illustrated in Figure 1-13 below.


Example from TIW L3 Presntation Window

Error protection scheme

Uplink Scrambling and Channelization codes

Downlink Channelization code

Figure 1-13 Example from TIW L3 Presentation Window

01/ 038 02-EN/LZU 108 6245 PA3 - 17 -


SPREADING AND SCRAMBLING EXAMPLE

An example of how a channel at 960ksps could be spread and scrambled by a transmitter is illustrated in Figure 1-14 below.


1 -1 1 1 -1Data

1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1C4,2

1 1 -1 1 1 1 -1 -1 11 -1 11 -1 -1 11 -1 -1 1 1 -1 -1SC

SF=4

1 -1 1 -1 1 -1 1 -1 -1 1 -1 1-1

1x-1=-1

1

1x 1= 1

-1

1x-1=-1

-1 1 -1 1-1x 1= -1

-1x-1= 1

-1x 1= -1

-1x-1= 1

Result 1

1x 1= 1

X

X

11 -1 -1 -1 -1 1 1 -1 1 1 1 1 -1 -1 -1 11 -1 1 1 -1

1x 1= 1

1x-1=-1

-1x 1= -1

Coded signal

Transmitter

Figure 1-14 Spreading and Scrambling Example

This data would be recovered in the receiver as illustrated in Figure 1-15 below.


Data

1 -1 1 -1 1 -1 1 -1 -1 1 -1 1-1 1 -1 11

-1x-1= 1

-1

-1x 1= -1

4 4 -4

Result 1

1x 1= 1X

1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1C4,2

1 1 -1 1 1 1 -1 -1 11 -1 11 -1 -1 11 -1 -1 1 1 -1 -1SC

11 -1 -1 -1 -1 1 1 -1 1 1 1 1 -1 -1 -1 11 -1 1 1 -1Coded signal

-1x1 = -1

-1

Data

X

1 x 1 = 1

1

-1x-1= 1

1

1 x 1 = 1

1

-1x-1= 1

1

1x-1=-1

-1

-1x1 = -1

-1

1x-1= -1

-1

-1x1 =-1

-1

Receiver

Figure 1-15 De-Scrambling and de-Spreading Example

- 18 - 01/ 038 02-EN/LZU 108 6245 PA3

1 WCDMA Transmitter

CHANNEL ADDITION

Each complex-valued spread and scrambled channel is separately weighted by a weight factor Gn. All downlink physical channels are then combined using complex addition. This is illustrated in Figure1-16 below.


Channel Addition

ΣG1

Gn Resulting symbol

CH1

CHn

Figure 1-16 Channel Addition

The resulting baseband symbols will still be at the chip rate and are applied to the RF modulator.

01/ 038 02-EN/LZU 108 6245 PA3 - 19 -


MODULATION

The type of modulation used in the WCDMA downlink is Quadrature Phase Shift Keying (QPSK). The baseband symbols are split into real and imaginary streams, filtered and applied to the I and Q inputs of the modulator. The I and Q branches modulate the RF carrier represented by Cos (ωt) and - Sin (ωt) respectively as illustrated in Figure 1-17 below.


Quadrature Phase Shift Keying (QPSK)

Pulse shaping

Pulse shaping

+

Cos (ωt)

- Sin (ωt)

Real

&

Imag

split

+ Cos (ωt)Or

- Cos (ωt)

+ Sin (ωt)Or

- Sin (ωt)

+ 1Or- 1

+ 1Or- 1

RF output (2Ghz)

Baseband Input

(3.84Mcps)

I

Q

Figure 1-17 Quadrature Phase Shift Keying (QPSK)

After modulation the I branch will swing between Cos (ωt) and - Cos (ωt) and the Q branch between Sin (ωt) and - Sin (ωt). These are combined to produce a waveform that swings between four phases.

- 20 - 01/ 038 02-EN/LZU 108 6245 PA3

1 WCDMA Transmitter

The scenario where I and Q are both positive is illustrated in Figure 1-18 below.


Modulation Example ( I =+1, Q=+1)

Q Branch

I Branch

Sum

Output

45o

Figure 1-18 Modulation Example ( I =+1, Q=+1)

The four possible combinations are illustrated in the constellation diagram shown in Figure 1-19 below.


QPSK Constellation Diagram

1 Modulation Symbol represents 2 data bits

Modulation efficiency = 2 bits/ symbol

I

Q

( I = 1, Q = 1 )

( I = -1, Q = -1 )

( I = -1, Q = 1 )

( I = 1, Q = -1 )

RF Carrier amplitude

RF Carrier phase angle

Figure 1-19 QPSK Constellation Diagram

This modulation efficiency of QPSK is two bits per modulation symbol.

01/ 038 02-EN/LZU 108 6245 PA3 - 21 -


Intentionally Blank

- 22 - 01/ 038 02-EN/LZU 108 6245 PA3

WCDMA Transmitter

Documents

Transcript of WCDMA Transmitter